1. Field of the Invention
The invention relates generally to polyenureas and methods of making them.
2. Description of the Related Art
Ketimines or blocked amines are known in the coating industry as curing agents for epoxy resins. Ketimines are the reaction products of amines or diamines with ketones splitting off water. The reaction is completely reversible, and in contact with humidity of the atmosphere the ketimines pick up water and split off the ketone (blocking agent) and the resulting free amine groups will react with the epoxy resin. This means that curing of epoxies with ketimines always results in the release of substantial amounts of volatile materials.
Aldimines are similar to the ketimines except that an aldehyde such as butyric aldehyde is used as the blocking agent. The reverse reaction with atmospheric humidity is less pronounced and as such aldimines are not used as curing agents for epoxies.
When aldimines or ketimines are added to isocyanate curing agents they react in a very similar way as with epoxy resins. They pick up water and the blocking agents are slowly released. The free amines then subsequently react with the isocyanate groups to form ureas. The reaction is however cumbersome as traces of free amines in the ketimine/aldimine make mixing a rather difficult exercise. The curing is also typified by a kind of skinning, i.e. a very strong skin forms at the surface, which protects the underlying mixture from further curing. It takes many days to fully cure isocyanates this way. This reaction mechanism can be typified as a moisture curing reaction and again splits of substantial amounts of volatile material.
Zwiener et al. (U.S. Pat. Nos. 5,126,170 and 5,236,741. All referenced patent documents and publications are incorporated herein by reference) describe the use of aldimines up to a level of 5 to 10% as a catalyst for curing polyols with isocyanates giving a long pot life and excellent mechanical properties. However, this patent does not deal with the use of totally solvent free coatings. It mentions the use of aldimines as a catalytic agent in the formation of polyurethanes.
Lee et al. (“Aldimine-Isocyanate Chemistry: A Foundation for High Solids Coatings,” Waterborne, Higher-Solids, and Powder Coatings Symposium (Feb. 22-24, 1995)) points out that at elevated temperatures (80° C.) there is a kind of tautomerism taking place whereby the aldimines form enamines, a form of unsaturated amines. These enamines subsequently react with isocyanate groups to form the corresponding polyenurea, a new class of polyureas. Lee also states that several other reaction paths do exist forming different ring structures whereby one aldimine group reacts with either one or two isocyanate groups, and suggests the use of aldimines as co-reagents in order to increase the solids of polyurethane coatings.